Black and Blue is my fav. Simple simple menu. All sorts of cuts of steak, served with salad and chips. There are locations in Kensington (between High Street Ken and Notting Hill Gate), Belsize Park, Gloucester Road and Marylebone.
Other good options are the Argentinian steak places - Gauchos or Guacho Grill. I know there is one in Hampstead and one near Liverpool Street Station.